Nvidia will call it strategy but the AIBs (Add in Board partners) were not that happy about Nvidia decision to postpone the launch of G84 and G86 chips. The chips are ready and the launch delay is mainly a strategy decision.
At least G84 chips are ready and the cards entered a volume production but Nvidia still decided to wave the flag and give its partners a red light for launch.
R600 got delayed again and as this is one of the last days of Cebit even the most optimistic lads realized that the new chip won't be launch in Q1 2007.
This lead Nvidia to push its mainstream DirectX 10 launch for a later day and enjoy the great sales of G80 based Geforce 8800 GTX and GTS cards for a while. The margins are great but the sales will slow once Nvidia introduce Geforce 8600 GTS / GT cards.
So that is the final answer why Nvidia delayed and of course Geforce 8600 GT cards have a minor bug that should be solved prior the launch. Now you know.